This invention relates to a drive system for a mobile platform having exceptional dynamic maneuvering capability.
Various drive systems have been employed to improve the maneuverability of a mobile platform. Where a small turning radius is required, the usual combination of drive wheels and separate steerable wheels is abandoned. A shorter turning radius is achieved by combining the drive and steering functions to common wheels. An example of this is a bulldozer steel tread drive where the track on one side is driven in reverse while the track on the other side is driven forward. The riding lawnmower industry has utilized small turning radius platforms. Typically, the two rear wheels are independently driven drive wheels and the front wheels are castor wheels. The drive wheels are operational in various combinations of forward or reverse drive to control direction of travel. The castor wheels allow the platform to turn under control of the rear wheels. Both friction drive and hydrostatic drive systems have been employed. U.S. Pat. Nos. 3,306,132 and 3,410,156 to Davis show a drive unit for a friction drive platform. Known drive systems make no controlled provisions for drive slippage, during dynamic turns, between the two drive wheels. In the case of a platform for a lawnmower, one or both of the drive wheels may slide with respect to the ground and causes the wheel to mar the ground.
A second effect is the large amount of energy required to execute a turn. Each drive wheel is simultaneously attempting to apply a force to a moving body. During a substantial portion of the turn, the force applied to the non-pivot wheel is in the wrong direction to obtain an optimum short turn, even though the direction of travel is correct. A substantial energy change is required with resulting delays in the responsiveness of the turn.
A drive system according to the present invention provides torque limited wheel drive to prevent wheel slippage with the travel bed and energy conserving turning capabilities to greatly accelerate the execution of a turn.
The presently disclosed apparatus includes a riding lawnmower platform and drive system which provides control from a hydraulic drive system to execute a dynamic right angle turn at the mowing path. The presently disclosed embodiment illustrates provision for direct manual operation or for an encoded control to be utilized by a remote control point. The encoded control point is also adaptable to a self-controlled steering device where provisions are provided on the mowing platform for sensing a guidance path and translating the information to a drive system control without manual assistance. The platform consists of two rear drive wheels and two castor wheels at the front. One or more castor wheels could be employed for other platform configurations. The swivel wheels are double swivel action castor wheels similar to that shown in U.S. Pat. No. 2,478,035 to Babcock. The double swivel action provides freedom of motion to the platform during abrupt turning maneuvers and enhances the turning response.
The hydraulic drive system and mobile platform embodiment presently disclosed are designed to provide equal drive speed to both hydraulic motors when steering correction control is not employed resulting in equal drive speed to the rear wheels on a platform employing this drive system. Since the drive speeds are locked together, the platform would track straight even on a steep bank. Three degrees of turning control are illustrated. The least degree is effected by applying a braking force of relatively low torque to the pivot drive wheel, while driving the other drive wheel at full speed. This is the only turning maneuver where a fixed drive torque is applied simultaneously at both drive wheels during the start of a turn. The other two degrees of a turn, medium and full turn, provide for a reverse direction torque to be applied at the pivot wheel while allowing the other wheel to free wheel when the momentum of the platform attempts to force this wheel in excess of full speed. When or if this wheel attempts to slow down to less than its full speed, a forward drive torque would then again be applied. During condition of medium or full turn, reverse torque is never applied to the non-pivot wheel. A torque limited brake is applied to the pivot wheel during medium turns. The pivot wheel has a torque limited reverse drive applied during full turn maneuvers.
The hydraulic controls are shown to be solenoid driven which enable them to be operational from an electrical device. An electrically interfaced manual control is also defined. Direct manual control of the hydraulic valves and controls could be implemented to accomplish the function described herein. The electrical control interface permits adaptation of the hydraulic system to electronic control. The electronic control provision permits rapid and frequent adjustment of the controls.